1. Field of the Invention
This invention relates to a confocal laser scanning mode interference contrast microscope, a method of measuring minute step heights on an object, and an apparatus using the microscope therefor.
2. Related Background Art
A confocal laser scanning microscope has the advantage that its depth of focus is very shallow, and is going to be used for various applications. Specifically, it has a laser source, an illuminating optical system for condensing a light beam from the laser source on an object to be examined and forming a light spot, a condensing optical system for condensing a light beam from the object to be examined on a detecting surface, detecting means for detecting the light team condensed on the detecting surface, and scanning means for moving the light spot relative to the object to be examined. The microscope condenses the laser beam on the object to be examined and detects the light on the detecting surface as well through a pin-hole opening.
To obtain a differential interference contrast image by the use of such a confocal laser scanning microscope, use can be made of the construction of a differential interference device in a conventional popular optical microscope. However, this is a complicated construction and moreover, requires a special objective lens of little distortion, a Nomarski prism, a wavelength plate, etc. The manufacture of various optical elements at high accuracy is difficult, and this has led to a high cost of the apparatus.
Generally, there have been utilized scan-type electron microscopes or probe-type microscopes in order to quantifiably measure minute step heights formed on an object in nanometer order. Also, atomic force microscopes (AFM) have begun to be utilized recently.
However, the scan-type electron microscopes and atomic force microscopes are expensive and the handling of those is difficult. In the probe-type microscopes, a probe is brought into contact with the surface of the object for the measurement, so the object is liable to be damaged and the probe is often broken, necessitating replacement of the probe often. Consequently, it is also disadvantageous from the viewpoint of the cost.
Therefore, the inventors of the present invention utilized a phase contrast microscope and a differential interference contrast microscope and tried to obtain the amount of step heights from image outputs of these microscopes quantifiably. This method is not impossible in principle but it is very difficult to establish a formula for the method from the wave optical image formation theory. Also, the principle for measuring step heights by utilizing the differential interference contrast microscope is disclosed page 70 in O plus E published in October, 1992, wherein a signal corresponding to the slope of the step structure is obtained when the width (not height) of the step structure on the object (phase object) is sufficiently larger than a light spot. That is, the method is applicable to the object having the step heights with gentle slopes, in which the measurement of the step heights can be performed without taking the effect of diffraction into consideration. Also, the effect of diffraction of the optical system is not taken into consideration. Accordingly, the resolution for an object having minute and narrow step heights is considerably reduced.